DESCRIPTION (Applicant's Abstract): The aim of the Biosocial Development Unit is to conduct studies on the development and microevolution of aggressive behavior in mice that have been selectively bred for the expression or inhibition of aggressive behavior. The specific aims in this Unit are: 1. To examine how genetic background, neonatal stimulation, and postnatal environments interact to produce stable inter-individual differences in adult social and maternal behaviors. 2. To examine how these same factors affect physiological systems using advanced techniques of neurobiological analysis and non-invasive optical brain imaging, and to relate these effects to bi-directional changes in behavioral systems. 3. To identify biological and environmental conditions under which the transfer of stable patterns of adaptation from parents to offspring is either favored or attenuated; To clarify possible windows of change in development and across generations which are related to comparable studies of human ontogeny and intergenerational development; and 4. To identify candidate genes, using biological, biochemical, and molecular techniques, that may modulate experiential influences in the development of high and low-aggressive mouse lines.